1. Field of the Invention
The present invention relates to an image forming technique using printing materials (color materials) of a plurality of colors inclusive of a special color.
2. Description of the Related Art
In a case where the printing materials used in a printing apparatus are inks or toners of the colors cyan (C), magenta (M), yellow (Y) and black (K), for example, image processing for generating print data applies color separation to RGB image data to separate the data into C, M, Y, K data corresponding to the respective printing materials. Color separation usually is carried out using a look-up table (LUT). In other words, the LUT defines a method of generating C, M, Y, K data indicating how printing materials are to be combined for a combination of items of R, G, B data.
Furthermore, there are cases where use is made of a special color for the purpose of enlarging the gamut that can be reproduced by a printing apparatus, e.g., for the purpose of reproducing a higher saturation, as a printing technique employed in a printing apparatus using ink or toner. That is, there are cases where in addition to the usually used C, M, Y (so-called “basic colors”) and K, use is made of printing materials of the colors red, orange, green, blue and violet as the special colors.
As a printing technique using such a special color, the specification of Japanese Patent Laid-Open No. 2001-136401 (Patent Document 1) proposes a technique for performing color separation by dividing a gamut into sub-gamuts that are based upon a combination of the color K and three chromatic colors and then extracting a combination of colors from each sub-gamut.
Further, the specification of Japanese Patent Laid-Open No. 2003-011432 (Patent Document 2) proposes a technique for performing color separation using both a special color and basic colors in a case where, in terms of the hue of a special color, saturation falls below that of the combination of basic colors in a certain brightness region when only a special color is used. Specifically, in a case where use is made of, e.g., green ink having a high brightness, the gamut is enlarged in a low-brightness portion of the hue of green by using a combination of cyan and yellow.
All of such conventional printing techniques can enlarge the gamut of a high-brightness, high-saturation region by performing a color separation in which a special color and other color materials are combined appropriately.
In the technique described in Patent Document 1, however, since a combination of colors is extracted from a sub-gamut of a combination of four colors, the combination is less than four colors and is limited to a combination that was defined at creation of the sub-gamut. A problem, therefore, is that it is impossible to obtain optimum color separation with regard to all gamuts.
Further, in the technique described in Patent Document 2, if a color material of a special color having a high brightness is used, a problem is that the gamut cannot be enlarged satisfactorily in a low-brightness region or in the vicinity of black.